FIG. 1 shows a model for explaining the function of a magnetostatic wave device. This magnetostatic wave device model has a magnetic garnet film 1 through which excited magnetostatic waves propagate, an RF signal feeder line 4 formed on one main surface 1a of the magnetic garnet film 1, a ground conductor 31 formed on the upper side of the magnetic garnet film 1 and the RF signal feeder line 4 through a dielectric 21, and a ground conductor 32 formed on the lower side of the magnetic garnet film 1 through a dielectric 22. In this model, when an external magnetic field Hex is applied in the negative direction of the X axis and when RF signals are fed to the RF signal feeder line 4, electromagnetic waves generated are converted to magnetostatic waves due to magnetic coupling, and the magnetostatic waves (magnetostatic forward volume waves) propagate through the magnetic garnet film 1 so as to be parted and to move apart from each other in the positive and negative directions of the Y axis. The dielectrics 21 and 22 in an actual device are constituted, e.g., of a non-magnetic garnet substrate or air. The dielectric constants of these two dielectrics may be the same or different.
In the above magnetostatic wave device model, when the magnetic garnet film 1 has a thickness d and a specific magnetic permeability .mu.r, when the distance between one main surface 1a of the magnetic garnet film 1 (surface on which the RF signal feeder line 4 is formed) and a surface 31a of the ground conductor 31 opposing thereto is t.sub.1, and when the distance between the other main surface 1b of the magnetic garnet film 1 and a surface 32a of the ground conductor 32 opposing thereto is t.sub.2, the magnetostatic waves satisfy the dispersion relation: ##EQU1## In the above expression, M.sup.2 =.mu..sub.r (k.sub.y.sup.2 +K.sub.z.sup.2)
N.sup.2 =K.sub.y.sup.2 +K.sub.z.sup.2 PA1 Ky=Wave number of magnetostatic wave propagating in the Y axis direction PA1 kz=Wave number of magnetostatic wave propagating in the Z axis direction ##EQU2## .omega..sub.h =.gamma..sup.. Hin Hin: Internal magnetic field intensity (A/m) PA1 .omega..sub.m =.gamma..sup.. (Ms) PA1 .omega.=2.pi.f PA1 .gamma.=2.21.times.10.sup.5 PA1 .mu..sub.0 =12.566.times.10.sup.-7 (H/m)
Ms: Saturation magnetization (Wb/m.sup.2) PA2 f: Frequency (Hz) PA2 Gyromagnetic ratio (m/A.sup.. s)
In the above expression, t.sub.1 and t.sub.2 may have any arbitrary values. In a conventional magnetostatic wave device, however, t.sub.1 and t.sub.2 have been said to be certain finite values and have been generally set, e.g., at t.sub.1 +t.sub.2 =about 1 mm, as will be described later.
FIG. 4 shows a configuration example of a conventional magnetostatic wave device. This configuration example has a pair of magnetic pole pieces opposing to each other, a semi-square-shaped ground conductor 3 sandwiched between magnetic pole pieces 7 and 8 and internal void 9, permanent magnets 11 magnetically connected to the magnetic pole pieces 7 and 8, a yoke 12 magnetically connected to the permanent magnets 11, a magnetic garnet film 1 disposed in the semi-square-shaped ground conductor 3, and a dielectric substrate 13 supporting the magnetic garnet film 1, an RF signal feeder line 4 being between the magnetic garnet film 1 and the dielectric substrate 13. Coils 17 are wound around the circumferences of the permanent magnet 11 and the magnetic pole piece 7.
The magnetic pole pieces 7 and 8 and the yoke 12 are constituted of materials such as iron, Permalloy, etc., and the semi-square-shaped ground conductor 3 is therefore disposed so as to surround the RF signal feeder line 4 for the purpose of blocking out electromagnetic wave radiation against the magnetic pole pieces 7 and 8 such that the feed signal suffers no loss at high frequency. The magnetic garnet film 1 is, for example, a YIG (yttrium iron garnet) film, etc., produced by a LPE (liquid-phase epitaxial) method.
An external magnetic field is applied to the magnetic garnet film 1 in the void 9 with a pair of the magnetic pole pieces 7 and 8. The external magnetic field is composed of a fixed magnetic field by the permanent magnets 11 and a variable magnetic field by the coils 17, wherefore the regulation of electric currents in the coils 17 controls the intensity of the external magnetic field applied to the magnetic garnet film 1. When a microwave signal or a quasi-microwave signal is fed to the RF signal feeder line (antenna) 4 formed of a microstripline, etc., an electromagnetic wave produced is converted to a magnetostatic wave having a frequency depending upon the intensity of the above external magnetic field, and propagates through the magnetic garnet film 1.
For example, a magnetostatic wave which propagates toward a side surface of the magnetic garnet film formed rectangularly is reflected from the side surface to go back in the counter direction. The magnetostatic wave having the same phase as the phase of the inputted electromagnetic wave is coupled and resonates in the rectangular magnetic garnet film. The wavelength which is the greatest of the wavelengths of the magnetostatic wave resonating at this time is twice, 2L, as large as the length L of the rectangular magnetic garnet film.
A magnetostatic wave device of a resonator structure using a magnetic garnet such as YIG shows a very large unloaded Q value (to be referred to as unloaded Q.sub.u, or simply as Q.sub.u, hereinafter) in a microwave band and a quasi-microwave band. It is thus expected that such device find some interesting applications. However, for a magnetostatic wave device to be practical, it is essential that the loaded Q value (to be referred to as loaded Q.sub.1, or simply as Q.sub.1, hereinafter) is large.
The referrence "A BANDPASS FILTER USING YIG FILM GROWN BY LPE", 1985 IEEE MTT-S Digest, Y. Murakami and S. Ito, Sony Corporation describes that when the above magnetostatic wave device is measured for an external Q value (to be referred to as external Q.sub.e or simply as Q.sub.e, hereinafter) while being operated as a band stop filter, the external Q.sub.e increases with a decrease in the thickness of a YIG film. The above reference uses a disc-shaped YIG film. The above reference describes nothing concerning loaded Q.sub.1, while the relationship of external Q.sub.e, unloaded Q.sub.u and loaded Q.sub.1 is generally expressed by EQU 1/Q.sub.1 =1/Q.sub.u +1/Q.sub.e
and if the external Q.sub.e increases with a decrease in the thickness of a YIG film, loaded Q.sub.1 ought to increase, since unloaded Q.sub.u is constant regardless of the plane form and the thickness of a YIG film.
The present inventor has carried out a duplicate experiment using a rectangular magnetic garnet according to the method shown in the above reference, and has measured a loaded Q.sub.1. As a result, as can be presumed from the above reference, it has been found that the loaded Q.sub.1 increases with a decrease in the thickness of a magnetic garnet film. Further, it has been revealed that the loaded Q.sub.1 found in the measurement by the present inventor is appreciably smaller than the loaded Q.sub.1 calculated on the basis of 1/Q.sub.1 =1/Q.sub.u +1/Q.sub.e.
While it has been found that it is sufficient to decrease the thickness of a magnetic garnet film for an increase in loaded Q.sub.1, problems to be shown below are liable to occur when the thickness of a magnetic garnet film is decreased. For example, when the surface of a magnetic garnet film is polished for improving its flatness, the parallelism of the two main surfaces of the film becomes poor, and the film thickness is sometimes non-uniform. In this case, the ratio of a change in the film thickness, caused by the worsening of the parallelism, increases as the film decreases in thickness. If the thickness of a magnetic garnet film varies, the dispersion relation of the magnetostatic wave varies from one position to another. When the non-uniformity of the film thickness is large, therefore, standing wave as designed can be no longer generated, and the characteristics deteriorate. It is therefore desirable to increase the loaded Q.sub.1 by means different from the decreasing of the thickness of a magnetic garnet film.
Meanwhile, it is also essential to downsize a magnetostatic wave device. In the configuration shown in FIG. 4, the magnetic pole pieces 7 and 8, the void 9, the yoke 12 and the permanent magnets 11 constitute a magnetic field generator. A smaller space of the void 9 is desirable since the magnetic field generator can be decreased in size, as is well known. For example, the magnetostatic wave device described in JP-A-1-303901 is devised to narrow the distance Lg between the magnetic pole pieces 7 and 8 into 1.15 mm by using a conductor film having a thickness of 2 to 100 .mu.m to constitute its semi-square-shaped ground conductor. In this case, the height of the void 9, i.e., the distance between one opposing surface 3a and the other opposing surface 3b of the semi-square-shaped ground conductor 3 is approximately 1 mm.